Correct the symbol for perpendicular (#328)

Unicode has two separate symbols for "up tack" (ie `has_bot.bot`) and "perpendicular" (not yet used in mathlib). Having `\perp` produce the former is confusing.

Author: eric-wieser

src/abbreviation/abbreviations.json

@@ -546,7 +546,7 @@
     "parallel": "∥",
     "pa": "▰",
     "pm": "±",
-    "perp": "⊥",
+    "perp": "⟂",
     "^perp": "ᗮ",
     "permil": "‰",
     "per": "⅌",